Replacing wood pallets with plastic pallets has been a goal for many years. The advantages of the plastic pallets are many as compared to wood, including greater durability, lighter weight, more consistent dimensions, improved cleanliness, water resistance, higher residual value for recycling, and no nails which may damage products being supported thereon.
One major hurdle to overcome with plastic is the cost. Plastic pallets are more expensive than wood by three to five times. This cost can be offset by the number of trips or shipments that can be achieved with plastic versus wood pallets. Another major hurdle is the stiffness of plastic pallets. Racking loaded pallets in warehouses for up to 30 days is common, and the combination of low tensile strength and creep limit the use of plastic.
There are three conventional methods of overcoming these weaknesses. The first is to add reinforcement such as steel or a composite to the pallet. This generally adds significant cost and weight and complicates recycling of the pallet. The second is to make the pallet taller. This limits the height of product to be stacked on the pallet. The third is to use reinforced or engineered resins. Again, this adds significant cost and weight. All three obviously limit the acceptance of plastic pallets.
U.S. Pat. No. 3,580,190 provides a partial solution to the stiffness problem by attaching top and bottom sheets 22,24 to the structural network 23, as shown in FIG. 1 thereof. However, this solution does not resolve the bending stiffness problem because large lateral and longitudinal unsupported areas still exist, such as in areas 26, 37, 38, 49 and 50. In other words, this design merely further stiffens the support column areas 67, 68, 69, 97, 98, 99, 28, 30, 32, which already provide substantial stiffness merely as a result of their height. The weakness of this design is apparent in column 6, lines 60-71, where Fowler recommends the use of a material having a flexural modulus (or Young's modulus) greater than about 200,000 psi. Such a high modulus material is apparently required because the structure described does not provide significant resistance to deflection along the length and width of the pallet. High modulus materials add substantial cost to the pallet.
Further complicating the problem, modern pallets typically require large openings for receipt of pallet jacks. For example, the pallet shown in FIGS. 1-3 includes a top deck portion 16 supported on a plurality of support columns 18, which are attached to support rails 20, which form the bottom deck 19. Such structure cooperates to form two large openings 11,13 on each side of the pallet 10, as well as four bottom openings 15 formed in the lower deck 19. In this configuration, the rails 20 of the lower deck 19 are typically structurally weak, resulting in poor deflection stiffness. Such problems have proven very difficult to overcome because of the very thin nature of the lower deck 19. Similarly, the thin design of the top deck 16 results in the same deflection problem between columns 18.
Because pallets are exposed to significant abuse, any solution to the stiffness problem must not adversely effect the impact strength of the pallet.
Accordingly, a need exists for improving the stiffness of modern plastic pallets configured to receive a pallet jack, without reducing impact strength of the pallet.